The purpose of this project is to study genetic factors required for transformation by RNA-containing sarcoma viruses. The approach is to isolate morphologic revertants of sarcoma virus-transformed nonproducer cells and to analyze these for viral or cellular alterations that interfere with the expression of transforming functions of the sarcoma viral genome. By this approach, evidence concerning the number of sarcoma viral and cellular genes involved in the expression of transformation should be elucidated.